A man who walked into Ashfield police station with his family one night to make a confession in relation to a suspicious missing persons case was told by the lead investigator to come back another day, an inquest has heard.

Frank Mennilli, now an assistant police commissioner and commander of the South-West Metro region, has been forced to defend his actions in the mid-1990s when he was in charge of investigating the disappearance of Richard Sajko, 21, who vanished days before he was due to give evidence against a former friend in a criminal trial.

The inquest into Mr Sajko's suspected murder heard of errors and oversights but Mr Mennilli maintained there was nothing worthy of criticism, despite no arrests or breakthroughs after two decades.

No answers after 19 years: Richard Sajko. Photo: Supplied

Mr Sajko was last seen leaving his job at Avis Car Rentals in Mascot with a passenger on the night before Mother's Day in 1995.

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He was due to go on trial after being pulled over in a tow truck driven by his friend Sam Testalamuta with a stolen car attached.

The men gave conflicting explanations of how the stolen car came to be in their possession and Mr Sajko had revealed to his mother, Rozi, that Mr Testalamuta had been pressuring him to change his statement. Mr Testalamuta threatened to kill Mr Sajko and blow up his car, the inquest heard.

Despite this, Mr Mennilli formed the early conclusion that there was no evidence to suggest foul play and told Mrs Sajko her son had probably faked the threats and run away, the inquest heard.

''I certainly had a number of suspicions but I certainly had no evidence to support those suspicions,'' he told Glebe Coroners Court, adding that there was no ''direct evidence'' of violence to warrant a homicide investigation.

The inquest heard that statements were not taken from several crucial witnesses and a knife found outside Avis two weeks after the disappearance was never analysed. Mr Sajko's car was left on a Croydon Street for 11 days until police investigated.

An Avis colleague who saw two ''rough-looking men'' visit Mr Sajko on the last day he was seen alive told the inquest she was only interviewed in 2008.

When police searched the family home of an associate, John Tuiletufuga, in 1995, he confessed to his parents that he had shot a man and buried him near the beach using a stolen shovel.

Mr Tuiletufuga's parents dragged him to Ashfield police station in the middle of the night and Mr Mennilli was called but he told the family to come back the next day. When a statement was finally taken, Mr Tuiletufuga denied the confession. He has since been deported to New Zealand for criminal convictions.

On Wednesday, Mr Mennilli said he had no recollection of the late-night phone call and didn't know why Mrs Sajko was only told about the confession two years ago.

''If you want to criticise the investigation, so be it, but I can honestly say that everything possible was done at that time,'' Mr Mennilli said.